Polyaxial bone anchoring device

ABSTRACT

A polyaxial bone anchoring device is provided comprising a bone anchoring element having a shank to be anchored in a bone and a head; a receiving part coupled to the shank and configured to pivotably receive the head, and having a channel for receiving a rod and a longitudinal axis; a pressure member configured to be positioned in the receiving part; a locking member insertable into the channel; wherein the pressure member has a deformable portion; and wherein, when the locking member is advanced into the channel in the direction of the longitudinal axis, the locking member in moveable from a position where it contacts the pressure member resulting in a load applied to the pressure member that clamps the head, to a position where the locking member contacts the pressure member such that the deformable portion is deformed and the locking member contacts the rod and clamps the rod.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present disclosure claims the benefit of U.S. Provisional PatentApplication Ser. No. 61/658,120, filed Jun. 11, 2012, the contents ofwhich are hereby incorporated by reference in their entirety, and claimspriority from European Patent Application EP 12 171 525.4, filed Jun.11, 2012, the contents of which are hereby incorporated by reference intheir entirety.

BACKGROUND

1. Field of the Invention

The invention relates to a polyaxial bone anchoring device, inparticular for use in spinal or trauma surgery. The polyaxial boneanchoring device comprises a bone anchoring element with a shank to beanchored in a bone and a head. The head is pivotably held in a receivingpart and can be fixed at an angle by applying pressure onto it via apressure member. With the receiving part, the bone anchoring element canbe coupled to a stabilization rod that is placed into a channel of thereceiving part and that can be secured by a locking member. The pressuremember comprises a deformable portion. When the locking member isadvanced into the channel, first, the locking member contacts thepressure member resulting in a load applied to the pressure member thatclamps the head and thereafter the locking member contacts the pressuremember such that the deformable portion is deformed and the lockingmember comes into contact with the rod and clamps the rod. With such alocking member the head and the rod can be fixed in a sequential mannerusing a tool with a single drive portion that engages the lockingmember.

2. Description of Related Art

US 2003/0100896 A1 describes a polyaxial bone anchoring device whereinin one embodiment a single piece locking element is used to lock thehead and the rod simultaneously when the locking element is tightened.

U.S. Pat. No. 7,972,364 describes a locking assembly for securing a rodin a rod receiving part of a bone anchoring device that includes a firstlocking element and a second locking element. With the first lockingelement and the second locking element the head of the bone anchoringelement and the rod can be locked independently using a tool with twodrive portions.

U.S. Pat. No. 8,088,152 B2 describes an orthopedic retaining systemcomprising at least one bone screw which has a head part and a threadedshaft pivotably mounted thereon. A clamping element is mounted in thehead part, which can be pressed against the threaded shaft from itsupper side and, as a result, secure the threaded shaft relative to thehead part. A retaining bar is arranged in a receptacle of the head part.Further, a clamping device is provided on the upper side of the headpart, by means of which the clamping element and the retaining bar arepressed into the head part such that the threaded shaft and theretaining bar are secured in positions relative to the head part. Theclamping device comprises an elastically deformable pressure elementwhich is displaced into a clamping position during actuation of theclamping device. With such a configuration, upon actuation of theclamping device, the pressure element abuts first on the clampingelement and thereby secures the pivotable threaded shaft in position onthe head part while the retaining bar remains freely displaceable. Onlyupon further actuation of the clamping device the pressure element iselastically deformed thereby abutting on the retaining bar and securingthe retaining bar in position.

SUMMARY

It is the object of the invention to provide an improved polyaxial boneanchoring device that provides for simplified handling and a reliablefixation of the head and the rod.

In an embodiment of the invention, the polyaxial bone anchoring deviceallows a surgeon to lock the head of the bone anchoring element in thereceiving part and to lock the rod in a sequential manner using only asingle tool with a single drive portion. By this sequential lockingmechanism it is possible to first lock, or at least preliminarily clamp,the head and thereafter clamp the rod and finally lock the rod and thehead. Moreover, a full locking of the head and the rod can be carriedout and thereafter the fixation of the rod can be loosened to performadjustments of the rod.

As discussed regarding the above embodiment, because only a single toolwith a single drive portion and one-piece loading member is needed forperforming these steps, the use of the polyaxial bone anchoring deviceis facilitated.

In one embodiment, the polyaxial bone anchoring device comprises onlyfew parts. The pressure member and the locking member can be used withexisting receiving parts. Hence, existing receiving parts may beupgraded with a sequential locking mechanism to allow an independenthead and rod fixation. Thus, the number of parts of the polyaxial boneanchoring device that has the sequential locking mechanism is the sameas the number of parts of a polyaxial bone anchoring device wherein thehead and the rod are locked simultaneously.

The pressure member and the locking element are relatively easy tomanufacture.

BRIEF DESCRIPTION OF THE DRAWINGS

Further features and advantages of the invention will become apparentfrom and will be best understood by reference to the following detaileddescription reviewed in conjunction with the accompanying drawings. Inthe drawings:

FIG. 1 shows a perspective exploded view of the polyaxial bone anchoringdevice according to a first embodiment.

FIG. 2 shows a perspective view of the polyaxial bone anchoring deviceaccording to the first embodiment in an assembled state.

FIG. 3 shows a perspective view from the bottom of the pressure memberof the polyaxial bone anchoring device according to the firstembodiment.

FIG. 4 shows a top view of the pressure member shown in FIG. 3.

FIG. 5a shows a cross-sectional view of the pressure member, the sectiontaken along line A-A in FIG. 4.

FIG. 5b shows an enlarged view of a portion of FIG. 5 a.

FIG. 6 shows a perspective view from one side of the locking member ofthe polyaxial bone anchoring device according to the first embodiment.

FIG. 7 shows a top view of the locking member of FIG. 6.

FIG. 8 shows a cross-sectional view of the locking member, the sectionbeing taken along line B-B in FIG. 7.

FIG. 9 shows a cross-sectional view of the polyaxial bone anchoringdevice according to the first embodiment with a rod inserted, whereinthe section is taken in a plane perpendicular to the rod axis in a firstconfiguration of the locking member.

FIG. 10a shows a cross-sectional view of the polyaxial bone anchoringdevice as in FIG. 9, wherein the locking member is in a secondconfiguration.

FIG. 10b shows an enlarged view of a portion of FIG. 10 a.

FIG. 11a shows a cross-sectional view of the polyaxial bone anchoringdevice as in FIG. 9 with the locking member in a third configuration.

FIG. 11b shows an enlarged view of a portion of FIG. 11 a.

FIG. 12 shows a perspective exploded view of the polyaxial boneanchoring device according to a second embodiment.

FIG. 13 shows a top view of the pressure member of the polyaxial boneanchoring device according to FIG. 12.

FIG. 14a shows a cross-sectional view of the pressure member of FIG. 13along line C-C in FIG. 13.

FIG. 14b shows an enlarged view of a portion of FIG. 13.

FIG. 15 shows a perspective view from the bottom of the locking memberof the polyaxial bone anchoring device according to the secondembodiment.

FIG. 16 shows a cross-sectional view of the locking member according tothe second embodiment.

FIG. 17 shows a cross-sectional view of the polyaxial bone anchoringdevice according to the second embodiment with the rod inserted, whereinthe section is taken in a plane perpendicular to the rod axis and thelocking member is in a first configuration.

FIG. 18a shows a cross-sectional view of the polyaxial bone anchoringdevice according to the second embodiment with the locking member in asecond configuration.

FIG. 18b shows an enlarged view of a portion of FIG. 18 a.

DETAILED DESCRIPTION

A polyaxial bone anchoring device according to a first embodiment isshown in FIGS. 1 and 2. It comprises an anchoring element 1 having ashank 2 with a threaded portion and a head 3. The head 3 has aspherically-shaped outer surface portion and, on its side opposite tothe shank 2, a recess 4 for engagement with a tool. A receiving part 5is provided for coupling the bone anchoring element 1 to a rod 6. In thereceiving part 5, a pressure member 7 is arranged to exert pressure ontothe head 3 of the bone anchoring element 1.

Referring to FIGS. 1 and 2 as well as FIGS. 9 to 12, the receiving part5 has a top end 51 and a bottom end 52 and is of substantiallycylindrical construction with a longitudinal axis C extending throughthe top end 51 and the bottom end 52. Coaxially with the longitudinalaxis C, a bore 53 is provided extending from the top end 51 to apredetermined distance from the bottom end 52. At the bottom end 52, anopening 54 is provided, the diameter of which is smaller than thediameter of the bore 53. The coaxial bore 53 narrows towards the opening54, for example, with a spherically-shaped section 55 that provides aseat for the head 3. However, the section 55 can have any other shapesuch as, for example, a conical shape, that ensures the function of thehead 3 being pivotably held in the receiving part 5 similar to a balland socket joint.

The receiving part 5 further comprises a U-shaped recess 56 starting atthe top end 51 and extending in the direction of the bottom end 52. Bymeans of the U-shaped recess 56, two free legs 57, 58 are formed thatextend towards the top end 51 and define a channel for receiving the rod6. Adjacent to the top end 51, a portion with an internal thread 59 isprovided at the inner surface of the legs 57, 58. In the embodimentshown, the internal thread 59 is a flat thread having substantiallyhorizontal upper and lower thread flanks. Any other thread form can beused for the internal thread 59, however, a thread form that reduces oreliminates splaying of the legs is preferable. Below the portion withthe internal thread 59 the bore 53 comprises a slightly enlarged portion53 a which provides space for the expansion of a portion of the pressuremember 7 described below.

As shown in FIG. 1 and FIGS. 3 to 5, the pressure member 7 is of asubstantially cylindrical construction with a top end 71 and a bottomend 72 and a cylinder axis C extending through the two ends 71, 72 andwhen the pressure member 7 is in a mounted state, the cylinder axis C isthe same as or coaxial with the longitudinal axis C of the receivingpart 5.

The pressure member 7 is arranged in the receiving part 5 such that itstop end 71 is oriented towards the top end 51 of the receiving part 5and the bottom end 72 is oriented towards the bottom end 52 of thereceiving part 5. The pressure member 7 comprises a first cylindricalportion 73 adjacent the bottom end 72 with an outer diameter that isslightly smaller than the inner diameter of the bore 53 of the receivingpart 5 so that the pressure member 7 can be introduced into thereceiving part 5 from the top end 51 thereof. Adjacent to the bottom end72, a spherically-shaped recess 74 is provided that cooperates with aspherical outer surface portion of the head 3 of the bone anchoringelement 1. A coaxial through-hole 75 extends through the pressure member7 that allows access to the screw head 3 with a tool (not shown). Theend of the cylindrical portion 73 opposite to the bottom end 72 has arim 73 a from which a substantially cylindrically-shaped recess 76extends in the direction of the bottom end 72. The dimensions of therecess 76 are such that the rod 6 can be inserted and guided therein.

Two upstanding walls 77, 78 are provided on the rim 73 a, the free endsof which form the top end 71 of the pressure member 7. The walls 77, 78have a height such that together with the recess 76 they form a channelfor inserting the rod 6, wherein the depth of the channel is greaterthan the diameter of the rod 6. Hence, when the rod 6 is inserted, thetop end 71 of the pressure member 7 extends above an upper surface ofthe rod 6. The walls 77, 78 are substantially cylindrically-shaped withthe cylinder axis being the cylinder axis C of the pressure member 7.Each wall 77, 78 is divided by a longitudinal slit 77 a, 78 a that issubstantially parallel to the cylinder axis C. The outer diameter of thepressure member 7 in the area of the walls 77, 78 is slightly smallerthan the outer diameter of the pressure member 7 at the cylindricalportion 73. Hence, when the pressure member 7 is inserted into the bore53 of the receiving part 5 and the walls 77, 78 are located in thesection 53 a of the bore 53, there is a gap 79 between the inner wall ofthe bore section 53 a and the walls 77, 78, as shown in FIGS. 9 to 11 b.The slits 77 a, 78 a render the walls 77, 78 elastically deformable withrespect to the cylindrical portion 73 of the pressure member 7. Thismeans, that the walls 77, 78 can be bent radially outward and returnelastically to their upright position. Hence, the walls 77, 78 form adeformable portion of the pressure member 7. Adjacent to the top end 71,the wall portions 77, 78 comprise inclined inner edges 77 b, 78 b thatprovides an abutment for a locking member 8, which is described below.The incline may be, for example, around 45° with respect to the centralaxis C.

The locking member 8 will be described with reference to FIGS. 1 and 6to 8. The locking member 8 is a monolithic piece. In the embodimentshown, it is a set screw. The locking member has a top end 81 and abottom end 82 that faces the pressure member 7 when the locking member 8is inserted between the legs 57, 58 of the receiving part 5. An externalthread 83 of the locking member 8 cooperates with the internal thread 59of the receiving part 5. A coaxial recess 84 is provided at the bottomend 82. It shall be mentioned, that the recess 84 can be omitted and/ora coaxial through hole can be provided. Adjacent the top end 81, thereis an engagement portion 85 for a tool, which may be, for example,formed as a recess with longitudinal grooves, as a hexagon or otherpolygon recess, torx-shaped recess or may have any other shape making itsuitable as an engagement portion. Hence, with the engagement portion85, the locking member 8 has a single drive portion for a drive tool.

Adjacent the bottom end 82, there is a cylindrical portion 87 with anouter diameter that is only slightly smaller than the inner diameter ofthe pressure member 7 between the walls 77, 78 so that the cylindricalportion 87 may extend between the walls 77, 78. Between the cylindricalportion 87 and the external thread 83, there is a conical portion 88with a cone angle selected such that it corresponds to the angle ofinclination of the inclined edges 77 b, 78 b of the walls 77, 78. Thesize of the conical portion 88 in an axial direction is greater than thesize of the inclined edges 77 b, 78 b of the wall portions 77, 78, sothat when the conical portion 88 comes into engagement with the inclinededges 77 b, 78 b, a movement of the locking member 8 towards thepressure member 7 keeps the inclined edges 77 b, 78 b and the conicalportion 88 in contact over a certain length.

The parts of the bone anchoring device are made of a bio-compatiblematerial, for example, of a bio-compatible metal or a metal alloy, suchas titanium, stainless steel, nickel titanium alloys, such as Nitinol,or made of a bio-compatible plastic material, such as for example PEEK(polyetheretherketone). The parts can be made all of the same or ofdifferent materials.

In use, the receiving part 5 and the anchoring element 1 as well as thepressure member 7 are normally pre-assembled such that the head 3 ispivotably held in the seat 55 of the receiving part 5 and the pressuremember 7 is placed onto the head 3. Usually, at least two polyaxial boneanchoring devices shall be connected via the rod 6. After insertion ofthe bone anchoring elements 1 into the bone, the receiving parts 5 arealigned by pivoting the receiving parts 5 relative to the heads 3 andthen the rod 6 is inserted.

The locking procedure will be explained with reference to FIGS. 9 to 11b. First, as shown in FIG. 9, the locking member 8 is inserted into thereceiving part 5 with the bottom end 82 of the locking member facing therod 6. The locking member 8 is then further screwed-in until itscylindrical portion 87 extends into the space between the walls 77, 78and the conical portion 88 of the locking member 8 abuts against theinclined edges 77 b, 78 b of the walls 77, 78. Further advancement ofthe locking member 8 into the receiving part 5 presses the pressuremember 7 against the head 3 as shown in FIGS. 10a and 10b . By means ofthis, the head 3 is clamped in an adjustable angular position withrespect to the receiving part 5 and is held in this position by means offriction. The head 3 may be only slightly clamped or nearly completelylocked.

Then, as shown in FIGS. 11a and 11b , the further advancement of thelocking member 8 results in a counterforce that the locking member 8experiences from the pressure member 7. The walls 77, 78 are bentradially outward when the conical portion 88 presses onto the inclinededges 77 b, 78 b thereby moving the walls 77, 78 into the gap 79 that ispresent between the wall portions 77, 78 and the inner wall of the boresection 53 a (see arrows in FIGS. 11a and 11b ). This allows the bottomend 82 of the locking member 8 to move deeper into the channel where therod 6 is seated and finally contact and lock the rod 6. By thisprocedure, the whole polyaxial bone anchoring device is locked.

It is possible to correct the position of the rod 6 without looseningthe locking of the head 3. To achieve this, the locking member 8 isscrewed-back until the rod 6 becomes displaceable. This is possible dueto the resilient property of the deformable walls 77, 78 of the pressuremember 7. It may even be possible to fully remove the locking member 8and to carry out a complete revision of the locking of the polyaxialbone anchoring device.

A second embodiment of the polyaxial bone anchoring device will bedescribed with reference to FIGS. 12 to 18 b. The second embodimentdiffers from the first embodiment in the design of the pressure memberand of the locking member. Parts and portions that are identical to thefirst embodiment have the same reference numerals and the descriptionthereof will not be repeated.

The pressure member 7′ has a top end 71 and a bottom end 72 and asubstantially cylindrical portion 73′ that extends from the top end 71to the bottom end 72.

A U-shaped recess 76′ extends from the top end 71 to a distance from thebottom end 72. The depth of the U-shaped recess 76′ is greater than thediameter of the rod 6 so that when the rod 6 is inserted, the top end 71of the pressure member 7′ extends above an upper surface of the rod 6.Small coaxial cylindrical recesses 76 a, 76 b extends from the top end71 into the sidewalls of the channel formed by the U-shaped recess 76′.The recesses 76 a, 76 b contribute to rendering the upper portion of thepressure member 7′ deformable. At a distance from the top end 71 of bothsidewalls, a notch 107 a, 107 b in the outer surface extending in atransverse or circumferential direction relative to the central axis Cis provided. The contours of the notches 107 a, 107 b are substantiallyV-shaped, i.e. the widths of the notches 107 a, 107 b are increasingtowards the outer surface of the sidewalls. This renders the upperportion of the pressure member 7′ elastically deformable in such a waythat compressing the sidewalls of the pressure member 7′ in an axialdirection leads to a reduction of the height of the sidewalls.

The locking member 8′ according to the second embodiment differs fromthe locking member 8 of the first embodiment in that it comprises only acylindrical projection 87′ that projects away from the threaded portion83. The cylindrical projection 87′ has a smaller diameter than thethreaded portion 83 of the locking member, such that at the end of thethreaded portion 83 a ring-shaped abutment 108 is formed that comes intocontact with the top end 71 of the pressure member 7.

In use, as shown in FIG. 17, when the locking member 8′ is inserted intothe receiving part 5 and advanced towards the pressure member 7′, thering-shaped abutment 108 comes into contact with the top end 71 of thepressure member. Further advancement of the locking member 8′ pressesthe pressure member 7′ against the head 3 such that the head 3 isclamped by friction. Due to the counterforce experienced by the lockingmember 8′ from the pressure member 7′ when it presses the head 3 intothe seat 55, the further advancement of the locking member 8 presses theabutment 108 against the top end 71 of the pressure member 7′. Thereby,the deformable portions formed by the notches 107 a, 107 b are deformedand the notches 107 a, 107 b are narrowed. Hence, the cylindricalportion 87′ of the locking member 8′ comes into contact with the surfaceof the rod 6 and presses onto the rod 6 to fix the rod 6.

For a correction of the position of the rod 6, the locking member 8′ canbe screwed-back slightly to render the rod 6 displaceable again. Becauseof the spring function of the deformable portion of the pressure member7′, the pressure on the head 3 that holds the head 3 in position isstill present and the head remains clamped. Final tightening of thelocking member 8′ locks the whole assembly.

Various modifications of the previous embodiments are conceivable. Inparticular, the deformable portion can be realized also by resilientelements that are mounted onto the upper end of a pressure member. Insuch a case, the clamping force can be adjusted by selecting an elementwith a specific spring force. In addition, features of the differentembodiments described can be combined among each other.

For the polyaxial bone anchoring device any known polyaxial boneanchoring devices can be used that comprise a bone anchoring elementthat is pivotably received in the receiving part. In particular, boneanchoring devices, wherein the bone anchoring element is introduced fromthe bottom end of the receiving part into the receiving part, may beused. The head does not need to have a spherical shape. It can also haveflattened portions that may be used for restricting the pivotingmovement to a single plane. The receiving part may be formeddifferently, in particular, inside. It can also have an inclined loweredge to allow an enlarged pivot angle to a specific direction. For thebone anchoring element, any known bone anchors, such as screws, nails,with or without cannulation, can be used.

The connection between the locking member and the receiving part doesnot necessarily have to be a threaded connection, other connections maybe possible, such as for example, a bayonet connection.

The invention claimed is:
 1. A polyaxial bone anchoring devicecomprising: a bone anchoring element having a shank to be anchored to abone and a head; a receiving part configured to couple to the shank andto pivotably receive the head, the receiving part having a longitudinalaxis and a channel for receiving a rod; a pressure member configured tobe positioned in the receiving part and to exert pressure onto the headto lock the head in the receiving part; a locking member insertable intothe channel, the locking member comprising a top end and a bottom endconfigured to face the pressure member; wherein the pressure member ismonolithic and has a U-shaped channel with a top end and a bottom end,the U-shaped channel forming two sidewalls, wherein at least onesidewall comprises an elastically deformable portion; and wherein, whenthe head, the pressure member, and the locking member are in thereceiving part, the locking member is moveable in the channel along thelongitudinal axis between a first position wherein the locking membercontacts the pressure member resulting in a load being applied to thehead by the pressure member that clamps the head, and a second positionwherein the locking member contacts the pressure member such that theelastically deformable portion is deformed to reduce a distance betweenthe bottom end of the locking member and the bottom end of the U-shapedchannel of the pressure member for the locking member to come intocontact with the rod to lock the rod.
 2. The polyaxial bone anchoringdevice of claim 1, wherein the elastically deformable portion is locatednear the top end of the U-shaped channel of the pressure element.
 3. Thepolyaxial bone anchoring device of claim 2, wherein, when the lockingmember is in the second position, the elastically deformable portion isdeformed such that the sidewalls near the top end of the U-shapedchannel of the pressure element bend radially outward from thelongitudinal axis of the receiving part.
 4. The polyaxial bone anchoringdevice of claim 1, wherein the sidewalls of the U-shaped channel extendabove the surface of the rod when the rod is seated in the channel andwherein the deformable portion is formed on each of the sidewalls. 5.The polyaxial bone anchoring device of claim 1, wherein the deformableportion comprises at least one slit extending in a directionsubstantially parallel to the longitudinal axis rendering the deformableportion deformable.
 6. The polyaxial bone anchoring device of claim 1,wherein the deformable portion comprises at least one notch extending ina direction substantially transverse to the longitudinal axis.
 7. Thepolyaxial bone anchoring device of claim 1, wherein the locking membercomprises an inclined surface portion and the pressure member comprisesa corresponding inclined surface portion that is configured to contactthe inclined surface portion of the pressure member such that when thehead, the pressure member, and the locking member are in the receivingpart and the locking member is advanced towards the pressure member, anincreasing force is generated that deforms the deformable portion awayfrom the longitudinal axis.
 8. The polyaxial bone anchoring device ofclaim 1, wherein the deformable portion is a portion of the pressuremember that is deformed when the locking member presses against it suchthat the distance in an axial direction between the locking member andthe pressure member is reduced.
 9. The polyaxial bone anchoring deviceof claim 1, wherein the locking member is a single piece.
 10. Thepolyaxial bone anchoring device of claim 1, wherein the locking memberis a set screw.
 11. The polyaxial bone anchoring device of claim 1,wherein the bottom end of the locking member comprises a surface portionthat comes into contact with the pressure member without the lockingmember contacting the rod.
 12. The polyaxial bone anchoring device ofclaim 11, wherein the surface portion is ring-shaped.
 13. The polyaxialbone anchoring device of claim 1, wherein the receiving part comprises atop end and a bottom end, a bore extending from the top end to thebottom end and wherein the channel for the rod is formed by a recessadjacent the top end with a substantially U-shaped cross-section. 14.The polyaxial bone anchoring device of claim 13, wherein by the recesstwo open legs are formed and wherein an internal thread is provided onthe legs that cooperates with an external thread on the locking member.15. The polyaxial bone anchoring device of claim 13, wherein thepressure element is provided in the bore and wherein the pressureelement is sized relative to the bore such that substantially at thelocation of the deformable portion there is a gap between the pressureelement and the inner wall of the bore.
 16. A method of inserting apolyaxial bone anchoring device having a shank and a head; a receivingpart having a longitudinal axis and a channel, a monolithic pressuremember having a U-shaped channel with a top end and a bottom end, theU-shaped channel forming two sidewalls, wherein at least one sidewallcomprises an elastically deformable portion, and a locking membercomprising a top end, a bottom end and a U-shaped channel with a bottomend, comprising: pivotably receiving the head into the channel of thereceiving part and coupling the receiving part to the shank; anchoringthe shank into a bone; positioning the pressure member in the receivingpart; receiving a rod into the channel of the receiving part; insertingthe locking member into the channel with the bottom end of the lockingmember facing the pressure member; advancing the locking member furtherinto the channel along the longitudinal axis so that the locking membercontacts the pressure member resulting in a load being applied to thepressure member that clamps the head; and further advancing the lockingmember into the channel along the longitudinal axis so that the lockingmember contacts the pressure member such that the deformable portion isdeformed to reduce a distance between the bottom end of the lockingmember and the bottom end of the U-shaped channel of the pressure memberand the locking member is in contact with the rod and locks the rod.